An electrical cable for the transmission of high-frequency signals via signal lines in the cable, said cable comprising at least one dielectric profile which is adapted to receive strip-shaped conductors in the longitudinal direction of the cable, said conductors forming the signal lines. Below, signal line is taken to mean one or more conductors which together serve to propagate an electrical signal.
It is a well-known phenomenon when the signal frequency is high that the current tends to run in the surfaces of the conductors because of current displacement. Cables for this purpose and comprising strip-shaped conductors are e.g. called strip lines or microstrips. Examples of this are known from the U.S. Pat. Nos. 4,149,026 and 5,296,651.
The characteristic properties which are of importance for attenuation and crosstalk in a transmission line, can only be calculated theoretically when the transmission line has very simple geometrical cross-sectional shapes. In other cases, empirical determinations are required. As a main rule, however, it may be assumed that the attenuation is small when the line capacitance is small and when the surface of the conductors is large. Although this may be achieved by increasing the dimensions, it is undesirable both owing to space and owing to the consumption of material.
The object of the invention is to provide an electrical cable where materials may be saved, while obtaining good characteristic, electrical properties as well as small dimensions.
This object is achieved in that in cross-section the strip-shaped conductors have a main contour which extends partially around an effective centre of the magnetic field which is created by an electric current through a respective signal line, and that a predominant part of one side of a strip-shaped conductor engages a respective dielectric profile, while a predominant part of the opposite side of the conductor faces toward another material.
The strip-shaped conductors may comprise a metal foil which is secured to the dielectric profiles. Alternatively, the conductors may be established by vapour deposition.
In relation to the prior art, the invention involves the constructive innovation that the strips of conductive material previously used in strip lines or microstrips are no longer plane, but curve around a line in the longitudinal direction of the cable. This line corresponds to the effective centre of the magnetic field which is created by an electric current through a respective signal line. Seen in cross-section, either the predominant convex or predominant concave part of the conductor will engage and be supported by a dielectric profile, while the opposite side of the strip-shaped conductor faces toward another material. Thus, two dielectrics are provided, and the invention opens up quite new possibilities of configuring the geometrical relations of the strip-shaped conductors to the two dielectrics, so that the electrical properties of the line may be affected in dependence on how the strip-shaped conductor curves, seen in cross-section, and how the two dielectrics are positioned relatively to the curved surfaces of the conductor.
A very simple use of the invention is a coaxial cable with a central line. In relation to the prior art, a considerable saving in copper consumption is obtained, because the otherwise solid central conductor may be replaced by the curved strip-shaped conductors.
Generally, low attenuation is desired, and this may be achieved at a low capacitance. Preferably, the dielectric profile comprises a coupling part and a plurality of rails extending from the coupling part and adapted to support a strip-shaped conductor at a distance from the coupling part. Hereby, the strip-shaped conductors may be supported by the dielectric material such that it is possible to obtain a relatively small field line density and a great spacing between the conductors, seen along the field lines in the dielectric having the greatest dielectric constant. The dielectric surrounding the protruding rails has a lower dielectric constant than the rails, thereby making it possible to achieve a low capacitance.
The cross-sectional shape of the strip-shaped conductors may be adapted to the concrete needs.
The invention also involves the advantage that it is much easier to manufacture cables of the present type. Preferably for flat cables, the cable comprises at least two dielectric profiles which each have a first coupling portion and a second coupling portion contiguous therewith, said rails protruding from the first coupling portion into a cavity which is defined by said first coupling portion and the second coupling portion from a second, adjacent dielectric profile.
Cables of the present type are frequently provided with an electrical shield, and, according to the invention, shield conductors may be arranged on the coupling part of at least some of the dielectric profiles. By first coating the first coupling parts with a shield conductor and then connecting the dielectric profiles mutually, it may be ensured that the pairs of signal lines are predominantly mutually separated by several layers of shield conductors.
Where shield conductors are provided and the signals are carried in differential form, it is desirable to obtain a so-called balanced cable. Previously, it has ben very difficult to obtain balanced cables, but according to the invention it is now much easier because there are now more degrees of freedom with respect to the geometrical relations of the strip-shaped conductors to the two dielectrics, and with respect to how the two dielectrics are positioned relatively to the curved surfaces of the conductor.
When the rails may protrude from the first coupling part at different distances from the associated second coupling part, very low crosstalk may be obtained between two pairs of differential signals. The crosstalk to the surroundings may also be reduced in that the rails are constructed such that in the longitudinal direction each strip-shaped conductor alternately has a maximum and a minimum distance from the associated first coupling part, and such that the difference between the distances of the strip-shaped conductors to the first coupling part is mainly as great as possible. In a preferred embodiment of a flat cable, the coupling parts are uniform, so that the cable is cheaper to manufacture. Preferably, the adjacent coupling parts are fixed mutually by means of a surrounding sheath.
In many uses, in particular for flat cables, it will normally be the concave parts of the conductors which engage the dielectric profile. For coaxial cables, however, it is expedient that the predominant part of the convex surface engages respective dielectric profiles, while a predominant part of the opposite surface faces toward the second dielectric which surrounds the rails that protrude from the previously mentioned first coupling portion.
According to the invention, a coaxial cable may be composed of a plurality of dielectric profiles whose coupling parts together form a cylindrical cable part. The cylindrical cable part will normally be enclosed by an electrical shield.
The rails, which protrude from the first coupling portion of the dielectric profiles, may extend radially inwards toward the centre of the cable, but may also have other directions. The rails, at the end, preferably have a trough-shaped part to support a curved strip-shaped conductor, so that in cross-section the conductor follows an arc of a circle coaxially with the centre of the cable. This leaves many cavities between the contact parts of the dielectric profiles, which carry the shield conductors, and the signal conductors, thereby reducing the effective dielectric constant so that the cable exhibits less attenuation.
Alternatively, the attenuation may be maintained at the usual level so that the low effective dielectric constant allows the cable to have a smaller external diameter.
In a cable type of the last-mentioned kinds, the conductors on the trough-shaped parts may be interconnected so that together they form the central line of the coaxial cable, which thus has the shape of a very thin cylindrical shell. Alternatively, the individual conductors may be insulated from each other for carrying several respective signals.
According to the invention, it is possible to achieve very compact cables with unsurpassed relationships between attenuation and crosstalk. Balanced cables can be obtained, and material can be saved.